This invention relates to the catalytic oxidation of sulfur dioxide to sulfur trioxide and to monolithic or honeycomb catalysts for the oxidation reaction. The invention particularly relates to improved monolithic catalysts and to sulfur acid manufacturing processes in which monolithic catalysts are used in preliminary contact stages and a particulate catalyst is used in the final stage.
Sulfuric acid is typically produced by catalytic gas phase oxidation of sulfur dioxide to sulfur trioxide [Eq. (1)] followed by hydration of the sulfur trioxide product to form sulfuric acid [Eq. (2)]. EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3 ( 1) EQU SO.sub.3 +H.sub.2 O .fwdarw.H.sub.2 SO.sub.4 ( 2)
Eq. (1) proceeds at useful rates over solid particulate catalysts containing alkali-vanadium or platinum active phases. Typical gas concentrations of SO.sub.2 at the inlet to the first pass of catalyst range from 4 to 13%. With adiabatic operation of each pass of the converter, four passes of catalyst are generally necessary to achieve overall SO.sub.2 conversions in excess of 99.7%. Heat exchangers precede each pass in order to cool the gas stream to the desired inlet temperature to the catalyst bed. Conversions of at least 99.7% of the original SO.sub.2 concentration are obtained through a double absorption design in which SO.sub.3 is removed from the gas stream through acid irrigated absorption towers that follow the second [2:2 interpass absorption (IPA) design] or third (3:1 IPA design) pass of catalyst in the converter.
Various monolithic catalysts have been proposed for use in lieu of particulate catalysts in the contact process for the manufacture of sulfuric acid.
Monolithic catalysts are comprised of a ceramic honeycomb or other foraminous support having a high surface area substrate at the foraminal wall surfaces of the support, and a promoter and active catalyst phase on the substrate. Such high surface area substrate is provided, for example, by application of an alumina or silica washcoat to a honeycomb of mullite or the like. Alternatively, a mixture of high and low porosity silica powders is extruded to produce the honeycomb support, the high surface area silica providing the high surface area substrate at the surfaces of the foraminal walls of the honeycomb. This substrate generally exhibits both high surface area and high porosity. An active phase for the oxidation of sulfur dioxide is deposited on the substrate through adsorption of platinum from soluble precursor salts or impregnation of a porous substrate with soluble alkali and vanadium salts.
Platinum catalysts on both particulate and monolithic substrates have been suggested in the art for conversion of sulfur dioxide to sulfur trioxide. Platinum-containing active phases supported on particulates for the oxidation of SO.sub.2 are well known. Examples of platinum supported on silica gels for the catalytic oxidation of sulfur dioxide are given, for example, in U.S. Pat. Nos. 1,683,694, 1,935,188, and 2,005,412. In these patents a platinum precursor salt is typically impregnated onto the particulate support then treated with a reducing gas such as hydrogen sulfide in order to "fix" the platinum active phase on the support prior to calcination. Promoter materials have been previously used with platinum on particulate catalysts. U.S. Pat. No. 2,005,412 uses promoter materials that include the elements aluminum, manganese, iron, nickel, copper, bismuth, molybdenum, beryllium, vanadium, tin, and chromium. U.S. Pat. No. 2,200,522 includes promoter species with the elements arsenic, vanadium, magnesium, chromium, and iron. U.S. Pat. No. 2,418,851 reports the use of palladium with platinum on a magnesium sulfate or aluminum oxide carrier.
The deposition of a high surface area washcoat onto a low surface area ceramic honeycomb substrate is described in U.S. Pat. Nos. 2,742,437 and 3,824,196. It is generally known in the art that monolithic catalysts can be used at gas velocities higher than those used with particulate catalysts (i.e., greater than 120 standard linear feet per minute, SLFM) as a consequence of higher geometric surface area per unit volume, higher concentration of the active phase near the catalyst surface to minimize mass and heat transfer to and from the gas phase, and much lower pressure drop per unit volume. Both oxidation rate and pressure drop constraints are less stringent for monolithic catalysts than for conventional particulate catalysts. The performance properties intrinsic to the monolithic structure are compared in detail to those of particulate catalysts by J. P. DeLuca and L. E. Campbell in "Advanced Materials in Catalysis," J. J. Burton and R. L. Garten, Eds., Academic Press, New York, 1977, pages 312-318.
U.S. Pat. No. 3,518,206 describes the preparation of monolithic catalysts comprising a colloidal silica washcoat onto which is deposited active catalytic material selected from a group that includes elemental Pt. A variety of catalyst structures are described for a wide variety of applications, and numerous different active phase materials are described and exemplified. Examples in this patent describe coating a 20 to 40 mesh SO.sub.2 oxidation catalyst containing potassium, vanadium, iron, and silica on a honeycomb material through the use of colloidal silica (15%, 7 millimicron particle size). Another example describes dipping an aluminum honeycomb in a slurry of alumina and colloidal silica, and thereafter depositing platinum from chloroplatinic acid on the resultant alumina/silica coating. The catalyst so produced is said to be useful for oxidations of carbon monoxide to carbon dioxide, hydrogen to water, for the reduction of nitrogen oxides, and for various hydrogenation reactions.
U.S. Pat. No. 3,554,929 discloses preparation of monolithic catalysts with a high surface area coating derived from colloidal alumina. The monolith may be in the form of a honeycomb. Active catalysts, such as Pt, are deposited on the alumina coated support.
U.S. Pat. No. 4,098,722 describes a method for making a catalyst body from corrugated metal sheets of an aluminum-containing ferritic steel. An alumina washcoat carrier is applied to the catalyst body followed by deposition of an active catalytic material such as Pt.
U.S. Pat. No. 4,744,967 describes a process for exhaust gas purification that includes an oxidation catalyst for SO.sub.2. An example discloses that the oxidation catalysts consist of honeycomb bodies with a cell density of 100 cells per square inch (cpsi) and an .alpha.-Al.sub.2 O.sub.3 coating onto which finely distributed platinum is deposited at 70.8 grams of Pt per ft.sup.3. Sulfuric acid of 77-80% strength is produced in a single step by Passing an SO.sub.2 -containing gas over the catalyst at a space velocity of 7500/hr, 420.degree.-460.degree. c. inlet temperature, and 20 to 50 mg/m.sup.3 dust after an electrofilter.
German Pat. DE 39 10 249 discloses a process for the production of a catalyst for the oxidation of sulfur dioxide gas that includes V.sub.2 O.sub.5, a potassium salt, diatomaceous earth, and a sodium polyacrylate binder. Addition of water to this catalyst mixture allows it to be extruded to obtain a honeycomb-like form.
Meissner U.S. Pat. No. 4,539,309 describes catalysts for the oxidation of sulfur dioxide to sulfur trioxide that are prepared by dissolving vanadium pentoxidein an alkali solution, acidifying the solution with sulfuric acid, mixing the acidified solution with a carrier, molding or extruding the mixture, and drying and calcining the molding or extrudate. Working examples describe particulate extrudates having diameters of 6 mm.
Certain of the monolithic catalysts known to the prior art have been subject to thermal degradation, for example, by sintering of the active phase, at the temperatures of conversion of sulfur dioxide to sulfur trioxide. Initially highly active, they lose activity rapidly in commercial operation. Other catalysts provide a level of activity until contaminated by reaction byproducts or dusts contained in the reaction gases, but lack the chemical stability necessary for effective regeneration of the catalyst. Consequently, there has been an unfulfilled need in the art for monolithic catalysts which provide for high rates of conversion but are both thermally and chemically stable.
U.S. Pat. No. 3,259,459 describes a process for the production of SO.sub.3 using either vanadium or platinum catalysts. The SO.sub.2 -containing gas stream is partially converted to SO.sub.3 in the first pass, then the SO.sub.3 is absorbed through an interpass absorption step. In the subsequent pass the converted SO.sub.2 -containing gas stream from which the SO.sub.3 was removed is passed into the next catalyst bed at a temperature of at least 20.degree. C. below that in the preceding pass. Interpass absorption is a common practice in the art, as further illustrated, for example, by the disclosures of U.S. Pat. Nos. 1,789,460, and 3,142,536.
U.S. Pat. No. 3,963,423 discloses a high gas throughput process for the conversion of SO.sub.2 to SO.sub.3. Each pass of catalyst contains at least three catalyst trays that are arranged either horizontally or vertically beside one another.
U.S. Pat. No. 3,987,153 describes an integrated process for the reduction of SO.sub.2 emissions from a single absorption sulfuric acid plant consisting of multi-stage oxidation of SO.sub.2. In at least the final pass, a cesium-containing particulate catalyst is loaded. Following absorption of the SO.sub.3 from the gas stream, residual SO.sub.2 is scrubbed by means of aqueous hydrogen peroxide and/or sulfur-containing peroxy acids.
UK Pat. Appl. GB 2,081,239 describes a catalytic oxidation process for producing SO.sub.3 from SO.sub.2 that uses monolithic catalyst. An SO.sub.2 - and O.sub.2 -containing gas stream is passed through the monolithic catalyst at a superficial gas velocity of at least 500 actual ft./min. The monolithic catalyst has an open cross-sectional area of at least 50% with open gas flow passages of at least 50 per square inch of cross-sectional area.
German Pat. DE 39 11 889 describes a contact process for the production of sulfuric acid that uses a catalyst arranged in one or more layers. The catalyst has several honeycomb-shaped openings with equivalent diameters between 3 and 15 mm and an open volume ratio between 40 and 70%. The catalyst is contained in a tube from which branch passages between the catalyst layers are located. These passages remove hot converted gases to heat exchangers after which the cooled gases are returned to the next catalyst layer in the tube.
In an effort to achieve economies of scale, sulfuric acid plants often are built with capacities of 2000 to 3000 short tons (ST)/day (as 100% H.sub.2 SO.sub.4) The SO.sub.2 gas composition is in the range of 10 to 11% by volume or higher. This rate of production leads to relatively large diameter (often 30 to 40 feet or more) reactor vessels containing catalyst loadings on the order of 30 to 50 liters/short ton (L/ST) (as 100% H.sub.2 SO.sub.4) or more per pass. Current regulations on SO.sub.2 emissions levels from sulfuric acid plants often require that 99.7% or more of the SO.sub.2 fed to the first pass of the reactor be converted to SO.sub.3 On the basis of the prior art, there is an unfilled need for a sulfuric acid process that gives high rates of H.sub.2 SO.sub.4 production, affords significantly lower catalyst loadings in the upper passes, and at the same time, permits high levels of overall SO.sub.2 conversion that equal or exceed 99.7% in a four-pass process.